Rapid dissipation of glyphosate in small forest ponds

Abstract

Glyphosate was applied to the water surface of four small boreal forest ponds and sixin situ microcosms at a rate of 0.89 Kg a.i./ha. Water samples collected over a period of up to 255 days were analyzed for glyphosate and its primary metabolite aminomethylphosphonic acid (AMPA). Glyphosate dissipated rapidly from all ponds with first order half-lives ranging from 1.5 to 3.5 days. The slowest dissipation rate occurred in the pond with the most calcareous water and sediments. Glyphosate remained at or above the treatment concentration in microcosms containing only water but decreased rapidly in the presence of sediments. AMPA levels in ponds and microcosms were consistently low. Concentrations on microcosm wall samples were temporally variable, probably a result of adsorption to periphytic biofilms. Glyphosate in the sediments of treated microcosms generally increased with time during the period of observation. These results confirm that glyphosate dissipates rapidly from the surfaces waters of lentic systems, and suggest that sediment adsorption or biodegradation were the major means of glyphosate loss from the water column.